Wavelength-Shifting Sheet-Coupled Scintillation Detectors

ABSTRACT

The present specification describes an X-ray detector that includes at least one scintillator screen for absorbing incident X rays and emitting corresponding light rays, a wavelength shifting sheet (WSS) coupled with the at least one scintillator screen for shifting the emitted light rays, at least one wavelength shifting fiber (WSF) coupled with at least one edge of the WSS for collecting the shifted light rays, and a photodetector for detecting the collected light rays.

CROSS-REFERENCE

The present application is a continuation application of U.S. Pat.Application No. 17/061,340, entitled “Wavelength-Shifting Sheet-CoupledScintillation Detectors” and filed on Oct. 1, 2020, which is acontinuation application of U.S. Pat. Application No. 16/382,973, of thesame title, filed on Apr. 12, 2019, and issued as U.S. Pat. No.10,830,911 on Nov. 10, 2020, which relies on, for priority, U.S. Pat.Provisional Application No. 62/687,550, entitled “Wavelength-ShiftingSheet Scintillation Detectors” and filed on Jun. 20, 2018. All of theabove referenced applications are herein incorporated by reference intheir entirety.

In addition, the present specification relates to U.S. Pat. ApplicationNo. 16/242,163, filed on Jan. 8, 2019, which is a continuation of U.S.Pat. Application No. 15/490,787, entitled “Spectral Discrimination usingWavelength-Shifting Fiber-Coupled Scintillation Detectors”, filed onApr. 18, 2017, which, in turn, is a divisional application of U.S. Pat.No. 9,658,343 (the “‘343 patent”), of the same title filed on Feb. 23,2016 and issued on May 23, 2017. The ‘343 patent is a continuation ofU.S. Pat. No. 9,285,488 (the ‘488 patent), of the same title, filed onFeb. 4, 2013, and issued on Mar. 15, 2016. The ‘488 patent, in turn,claims priority from the following applications:

U.S. Pat. Provisional Application No. 61/607,066, entitled “X-RayInspection using Wavelength-Shifting Fiber-Coupled Detectors”, filed onMar. 6, 2012.

U.S. Pat. Provisional Application No. 61/598,521, entitled “DistributedX-Ray Scintillation Detector with Wavelength-Shifted Fiber Readout”, andfiled on Feb. 14, 2012.

U.S. Pat. Provisional Application No. 61/598,576, entitled “X-RayInspection Using Wavelength-Shifting Fiber-Coupled Detectors”, and filedon Feb. 14, 2012.

The above-mentioned applications are incorporated herein by reference intheir entirety.

FIELD

The present specification relates generally to wavelength-shiftingscintillation detectors and more specifically to a wavelength-shiftingsheet detector for use in a flying spot transmission imaging system.

BACKGROUND

Fiber-coupled scintillation detectors of radiation and particles havebeen employed over the course of the past 30 years. In some cases, thescintillator is pixelated, consisting of discrete scintillator elements,and in other cases, other stratagems are employed (such as orthogonallycrossed coupling fibers) in order to provide spatial resolution.Examples of fiber-coupled scintillation detectors are provided by U.S.Pat. Nos. 6,078,052 (to DiFilippo) and 7,326,933 (to Katagiri et al.),both of which are incorporated herein by reference. The detectorsdescribed by both DiFilippo and Katagiri et al. employwavelength-shifting fibers (WSF) such that light reemitted by the corematerial of the fiber may be conducted, with low attenuation, tophoto-detectors disposed at a convenient location, often distant fromthe scintillator itself. Spatial resolution is of particular value inapplications such as neutron imaging. Spatial resolution is alsoparamount in the Fermi Large Area Space Telescope (formerly, GLAST)where a high-efficiency segmented scintillation detector employs WSFreadout for detection of high-energy cosmic rays, as described inMoiseev, et al., High efficiency plastic scintillator detector withwavelength-shifting fiber readout for the GLAST Large Area Telescope,Nucl. Instr. Meth. Phys. Res. A, vol. 583, pp. 372-81 (2007), which isincorporated herein by reference.

A conventional scintillation detector 100 is shown in a sidecross-section in FIG. 1A and in a front cross-section in FIG. 1B. Anexample of such a detector is described in U.S. Pat. No. 5,302,817 (toYokota), and is incorporated herein by reference. Typically, alight-tight box 102 is lined with scintillating screens 103 whereincident X-ray radiation 101 is converted to scintillation light,typically in the UV, visible, or longer wavelength, portions of theelectromagnetic (EM) spectrum. Large-photocathode-area photomultipliertubes (PMTs) 105 are coupled to receive scintillation light viaportholes 108. One problem lies in that a fraction of the scintillationlight originating within the screen is transmitted from the screen intothe enclosed volume. The remaining scintillation light is lost in thescreen material. Scintillating screens 103 are designed to maximize thefraction of emitted light, which is tantamount to ensuring a largetransmission coefficient T for the interface between screen 103 and themedium (typically air) filling the detector volume.

However, in a conventional backscatter detector of the sort depicted inFIGS. 1A and 1B, the scintillation screens 103 should also serve as goodreflectors because scintillation light, once emitted into the volume ofbox 102, typically needs multiple reflections until it reaches aphotodetector 105. So, the reflection coefficient R of the screensurface should also be large, however, since the sum of T and R isconstrained to be unity, both T and R cannot be maximizedsimultaneously, and a compromise must be struck. As a result, the lightcollection efficiency of the conventional backscatter detector isinherently low, with only a few percent of the generated scintillationlight collected into the photo detectors. Poor light collection canpossibly create a secondary quantum sink and increase image noise. Thelight collection efficiency can be improved by increasing the sensitivearea of the photo-detectors which is not only costly, but also addsweight and size. A conventional backscatter (BX) detector assembly withphotomultiplier tubes (PMT) power supplies weighs typically between 3and 4 g/cm². The light box is typically designed with an aspect ratio of1:10 for height to thickness, in order to minimize the number ofreflections from the internal surfaces. For typical sizes required fortransmission detectors in handheld applications, the light box would be2-3″ in thickness, with additional thickness requires for PMT mounting.In addition to size and light collection efficiency, conventional lightbox detectors are inherently non-uniform in response for application asa transmission detector. Response across the screen in the locationswhere the PMT is located show a significant drop in response due to thelack of scintillator material in the back of the light box in theselocations.

Detectors used in transmission imaging with a handheld flying spot X-rayscanning system may be constructed from materials which are far morethin and rugged than traditional flat panel detectors. For transmissionX-ray detection with a handheld scanning system, the detector may not berigidly attached to the body of the system. In this case, the detectoris required to be the same size as the object being imaged, in order tointercept the flying spot beam across the area of interest. In suchimaging configurations, the detector response may not be corrected orcalibrated due to the non-uniform and non-repeatable illumination of thedetector by the source. In this case, any non-uniformity in X-raysensitivity will be displayed directly in the final images.

Portable hand-held scanners currently utilize low-profile, light-weightWave-Shifting Fiber (WSF) X-ray detectors in order to generatetransmission X-ray images. In general, WSF detector technology enables alow profile, rugged and large area detection of a flying spot x-raybeam. By way of background, wavelength shifting fibers consist of a corewith relatively high refractive index, surrounded by one or morecladding layers of lower refractive index. The core containswavelength-shifting material, also referred to as dye. Scintillationlight which enters the fiber is absorbed by the dye which, in turn,emits light with a longer wavelength. The longer wavelength light isemitted isotropically in the fiber material. Total internal reflectiontraps a fraction of that light and conducts it over long distances withrelatively low loss. This is possible, as described with reference toFIG. 2 , because the absorption 204 and emission 202 wavelength rangesof the dye effectively do not overlap so that the wavelength-shiftedlight is not reabsorbed. The captured fraction is determined by theratio of the refractive indices at surfaces of the fiber. An additionaladvantage of WSF is that the wavelength shifting can bring thescintillation light 206 into the sensitive wavelength range of the photodetector (PMT, silicon photomultiplier (SiPM), or Multiple-PixelPhoton-Counter (MPPC), or otherwise).

FIG. 3A illustrates a known X-ray detector comprising WSF andscintillator layers, which is disclosed in co-pending U.S. Pat.Application No. 15/490,787, assigned to the Applicant of the presentspecification, which is herein incorporated by reference. U.S. Pat. No9,285,488 and 9,658,343, also assigned to the Applicant of the presentspecification, are herein incorporated by reference in their entirety.As shown, a layer of closely spaced parallel wavelength-shifting fibers300 is sandwiched between two layers 303 of composite scintillatingscreen. One commonly used scintillator material is europium-doped bariumfluorochloride (BaFCI:Eu), although other scintillators, such asBaFI:Eu, or other lanthanide-doped barium mixed halides (including, byway of further example, BaBrI:Eu and BaCsI:Eu), are also used. Compositescintillator 303 is structurally supported by exterior layers 304 ofplastic, or other material, providing mechanical support. Opticalcontact between the fiber cladding 301 and the composite scintillator303 is established by filling the voids with index-matching material 305of suitable refractive index which is transparent to the scintillationlight. The refractive index of the filling material is chosen tooptimize the collection of primary light photons into the WSF and thecapture of wavelength-shifted photons in the fiber. Filling material 305may be optical grease or optical epoxy, for example.

Upon incidence of X-ray photons, scintillation light emitted byscintillator 303 is coupled via cladding 301 into core 307 of therespective fibers, down-shifted in frequency (i.e., red-shifted) andpropagated to one or more photo-detectors, whereby the photo-detectorsconvert the light from the fiber cores 307 into a current. The currentis integrated for an interval of time, typically in the range of 1-12µs, to obtain the signal strength for each pixel. Integration of thedetector signal may be performed by an integrating circuit (not shown),such as an integrating pre-amplifier, for example. The useful stoppingpower of the detector can be increased by combining multiple layers ofWSF 300 thereby increasing the depth of scintillator material 303 alongthe path of the incident radiation. FIG. 3B illustrates across-sectional view of a typical WSF detector. As shown, a ribbon ofWSF is sandwiched between scintillator screens 303. The fiber ends arebundled, cut and polished. The exit surface is mounted to a PMT. Thisguarantees efficient light collection. In order to minimize the numberof PMT’s, the fibers can also be bent into a U-shape, and bundled at oneend of the detector.

One of ordinary skill in the art understands that the visibility of anartifact is a function of its size. Extended as well as abruptlychanging artifacts are highly visible in a noisy background. For a WSFdetector, a change in the efficiency of a single fiber (for instance asingle point defect in a fiber) results in an extended and abrupt linedefect. Such defects are highly visible as the defect extends across thelength of the detector. In addition, non-uniformity may occur during oras a result of the manufacturing process of the WSF fiber (such as, butnot limited to, cable bending, fiber bundling, and outputcoupling/polishing).

To overcome the challenges of achieving uniform response, a WSF detectormust be manufactured in such a way that maintains the fiber position andbending uniformly across the full detector. Any variations in thespacing or bending can lead to non-uniformity in the detector response.As a result, wavelength-shifting fibers must be physically held bymechanical fixture across the full surface of the detector, which maynumber in the hundreds. The fiber threading the handling constitutes amanufacturing challenge which adds cost and drops final quality andyield. Thus, there is a need for a WS detector configuration for use ina flying spot transmission imaging system with improved spatialuniformity and reduced cost for materials and manufacturing.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods, which aremeant to be exemplary and illustrative, and not limiting in scope. Thepresent application discloses numerous embodiments.

The present specification discloses an X-ray detector comprising: atleast one scintillator screen configured to absorb incident X rays andemit corresponding light rays; a wavelength shifting sheet (WSS)optically coupled with the at least one scintillator screen andconfigured to collect and spectrum shift the light rays, wherein the WSShas at least one edge; at least one wavelength shifting fiber (WSF)optically coupled with the at least one edge of the WSS and configuredto collect the spectrum shifted light rays and spectrum shift thecollected spectrum shifted light rays to generate twice-spectrum-shiftedlight rays; and a photodetector optically coupled to the WSF andconfigured to receive and detect the twice-spectrum-shifted light rays.

Optionally, the WSS comprises a first and a second surface wherein theat least one scintillator screen at least partially covers the firstsurface and a second scintillator screen at least partially covers thesecond surface. Optionally, the first surface is coplanar to the secondsurface.

Optionally, the at least one WSF is physically coupled with at least aportion of the edge of the WSS.

Optionally, the photodetector is a photomultiplier tube (PMT).

Optionally, the X-ray detector further comprises a reflector materialcovering the WSF to improve the collection of the spectrum shifted lightrays. Optionally, the reflector material comprises at least one of adiffuse reflector or a specular reflector material.

Optionally, the at least one scintillator screen comprises a materialhaving an optical absorption length and wherein a thickness of the atleast one scintillator screen is less than the optical absorptionlength.

Optionally, the at least one scintillator screen comprises BaFCI:Eu.

Optionally, the X-ray detector further comprises a spatially varyingattenuating material inserted between the at least one scintillatorscreen and the WSS, wherein the spatially varying attenuating materialis configured to correct a non-uniformity in detection by thephotodetector. Optionally, the spatially varying attenuating materialcomprises a plastic substrate printed sheet with absorbing ink on asurface of the plastic substrate printed sheet.

Optionally, the at least one scintillator screen is coupled with the WSSby placing the at least one scintillator screen over a surface of theWSS and wherein the at least one scintillator screen at least partiallycovers the surface.

The present specification also discloses an X-ray detector configured todetect X-rays, the detector comprising: at least one scintillator screenconfigured to absorb incident X rays and emit light rays based on theabsorbed incident X-rays; a first wavelength shifting sheet (WSS1)coupled with the at least one scintillator screen configured to shiftspectra of the light rays, wherein the WSS1 comprises at least one edge;a second wavelength shifting sheet (WSS2) coupled with at the least oneedge of said WSS1 and configured to collect the spectra shifted lightrays; and a photodetector configured to detect the collected spectrashifted light rays, wherein the photodetector is in opticalcommunication with the WSS2.

Optionally, the WSS1 comprises a first and a second surface, wherein theat least one scintillator screen partially covers the first surface anda second scintillator screen partially covers the second surface.Optionally, the first surface is coplanar to the second surface.

Optionally, the photodetector is a photomultiplier tube (PMT).

Optionally, the X-ray detector further comprises a reflector materialcovering the WSS2 to improve the collection of the spectra shifted lightrays. Optionally, the reflector material comprises at least one of adiffuse reflector or a specular reflector material.

Optionally, the at least one scintillator screen comprises a materialhaving an optical absorption length and wherein a thickness of thescintillator screen is less than the optical absorption length.

Optionally, the at least one scintillator screen is made of BaFCI:Eu.

Optionally, the X-ray detector further comprises a spatially varyingattenuating material inserted between the scintillator screen and theWSS, wherein the spatially varying attenuating material is configured tocorrect non-uniformity in detection photodetector.

Optionally, the spatially varying attenuating material comprises aplastic substrate printed sheet with absorbing ink on a surface of theplastic substrate printed sheet.

The aforementioned and other embodiments of the present specificationshall be described in greater depth in the drawings and detaileddescription provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present specificationwill be further appreciated, as they become better understood byreference to the detailed description when considered in connection withthe accompanying drawings:

FIG. 1A shows a side cross-sectional view of a “box-type” prior artscintillation detector;

FIG. 1B shows a front cross-sectional view of a “box-type” prior artscintillation detector;

FIG. 2 depicts spectral relationships among scintillation light andtypical wavelength-shifting fiber absorption and emission spectra asknown in the prior art;

FIG. 3A illustrates a prior art X-ray detector comprising WSF andscintillator layers;

FIG. 3B is a cross-sectional view of a prior art wavelength-shiftingfiber (WSF) based detector;

FIG. 4A is a diagrammatical representation of a WSF detector panel;

FIG. 4B illustrates a diagrammatical front and side view of the WSFfibers coupled in a 4x4 array and being held by a mechanical fixture;

FIG. 4C is an illustration of the WSF detector panel described withrespect to FIG. 4A;

FIG. 4D is a plot illustrating fiber-fiber response uniformity acrossthe prior art detector panel shown in FIG. 4A;

FIG. 5 is a pictorial representation of a bundled end of 1 mmwave-shifting fibers, which have been cut and polished for coupling to aPMT surface;

FIG. 6A illustrates a wavelength shifting sheet (WSS) detector, inaccordance with an embodiment of the present specification;

FIG. 6B is a side view of the WSS detector shown in FIG. 6A;

FIG. 6C is a diagrammatical representation of a WS detector, showinglight shifted by a second WS material in a direction perpendicular to anedge of a WS detector, in accordance with an embodiment of the presentspecification;

FIG. 6D is a pictorial representation of an image obtained by placing ascintillator screen close to the edge of the WSS;

FIG. 7A illustrates a diagrammatical side view of a WSS X-ray detector,in accordance with an embodiment of the present specification;

FIG. 7B illustrates a diagrammatical front view of the WSS X-raydetector shown in FIG. 7A;

FIG. 7C illustrates a diagrammatical side view of a WSS X-ray detector,wherein the thickness of the second WS material is increased, inaccordance with an embodiment of the present specification;

FIG. 7D is a block diagram of a segmented WS sheet of a WSS X-raydetector, in accordance with an embodiment of the present specification;

FIG. 7E illustrates a plurality of channels milled into the surface of aWS sheet, in accordance with an embodiment of the present specification;

FIG. 7F is a block diagram illustrating a front view of FIG. 7E, showingchannels milled into the surface of a WS sheet, in accordance with anembodiment of the present specification;

FIG. 7G illustrates a two-step spectral overlap in a segmented WSSdetector, in accordance with an embodiment of the present specification;

FIG. 8A is a graph depicting an absorption and emission spectra ofBaFCI:Eu in the wavelength shifting sheet layer of the detector shown inFIG. 7A;

FIG. 8B is a graph depicting an absorption and emission spectra ofBaFCI:Eu in the wavelength shifting fiber layer of the detector shown inFIG. 7A;

FIG. 9A illustrates light collection or acceptance cones in a wavelengthshifting fiber;

FIG. 9B illustrates light loss cones in a wavelength shifting sheet;

FIG. 9C is a diagrammatical representation of light collection cones ina rectangular WS fiber;

FIG. 9D is a graphical representation of the light capture efficienciesof a WS Sheet and a WS fiber made of materials having the samerefractive index;

FIG. 10A illustrates a WSS detector comprising a wavelength shiftingsheet coupled with a wavelength shifting fiber (WSF) wrapped around fouredges of the sheet, in accordance with an embodiment of the presentspecification;

FIG. 10B illustrates image data obtained from the detector shown in FIG.10A;

FIG. 10C illustrates the image response of the WSS detector shown inFIG. 10A;

FIG. 10D illustrates a log-linear cross-cut of the signal response shownin FIG. 10C;

FIG. 11A is a block diagram depicting an attenuating material insertedbetween WSS and a scintillator screen, in accordance with an embodimentof the present specification;

FIG. 11B is an image showing the response uniformity of the WSSdetector, in accordance with an embodiment of the present specification;

FIG. 11C is an image showing a compensating pattern of the attenuatingmaterial used in the WSS detector shown in FIG. 11A;

FIG. 12A is an image showing signal response from a wavelength shiftingfiber edge coupled WSS detector;

FIG. 12B is a graph showing the signal response of FIG. 12A, from ahorizontal cross-section through a center region of the WSS detector;

FIG. 12C is a graph depicting a wavelength shifting sheet detectorresponse with a variable number of fibers used for light collection atthe edge of the wavelength-shifting sheet, in accordance with anembodiment of the present specification;

FIG. 12D shows a plot depicting a WSS detector response with variablenumber of fibers used for light collection at the edge of the WS sheet,in accordance with an embodiment of the present specification;

FIG. 13A is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to foursides of a wavelength shifting sheet enabling a four side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification;

FIG. 13B is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/couple to twosides of a wavelength shifting sheet, enabling a two side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification;

FIG. 13C is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to oneside of a wavelength shifting sheet, enabling a single side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification;

FIG. 14A is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to foursides of a wavelength shifting sheet, enabling a four side as used in abackscatter detection mode, in accordance with an embodiment of thepresent specification;

FIG. 14B is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to twosides of a wavelength shifting sheet, enabling a two side readout asused in a backscatter detection mode, in accordance with an embodimentof the present specification;

FIG. 14C is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/couple to oneside of a wavelength shifting sheet, enabling a single side readout asused in a backscatter detection mode, in accordance with an embodimentof the present specification;

FIG. 14D is a diagrammatical representation of a rectangular WSSdetector being used in a backscatter detection mode, having eight WSfibers placed within a plurality of channels cut into a wavelengthshifting sheet, in accordance with an embodiment of the presentspecification;

FIG. 15A is a side view of a WSS detector comprising a plurality ofchannels for holding WS fibers, in accordance with an embodiment of thepresent specification;

FIG. 15B is a top-down view of the WSS detector shown in FIG. 15A,comprising a plurality of channels for holding WS fibers, in accordancewith an embodiment of the present specification;

FIG. 15C is a diagrammatical representation of a WSS detector array witha segmented, pixelated WS sheet comprising a plurality of channels forholding WS fibers, in accordance with an embodiment of the presentspecification;

FIG. 15D is a top view of the WSS detector array of FIG. 15C, showingtwo WS fibers placed within channels cut into the segmented WS sheet, inaccordance with an embodiment of the present specification;

FIG. 16 illustrates a curved WSS detector, in accordance with anembodiment of the present specification;

FIG. 17A is a diagrammatic representation of a WSS detector as used in atransmission mode with a flying spot X-ray imager, in an embodiment ofthe present specification;

FIG. 17B is a flow diagram representing the steps of using an WSSdetector in a transmission mode with a flying spot X-ray imager, inaccordance with an embodiment of the present specification;

FIG. 17C is a diagrammatic representation of a WSS detector as used in abackscatter mode with a flying spot X-ray imager, in an embodiment ofthe present specification;

FIG. 17D is a flow diagram representing the steps of using an WSSdetector in a backscatter mode with a flying spot X-ray imager, inaccordance with an embodiment of the present specification;

FIG. 17E is a diagrammatic representation of a multi-energy WSS detectoras used in a transmission mode with a flying spot X-ray imager, in anembodiment of the present specification;

FIG. 17F is a flow diagram representing the steps of using themulti-energy WSS detector shown in FIG. 17E, in a transmission mode witha flying spot X-ray imager, in accordance with an embodiment of thepresent specification;

FIG. 17G is a diagrammatic representation of a vertical WSS detector anda ground-level WSS detector as used in a transmission mode with a flyingspot X-ray imager for scanning a cargo object, in an embodiment of thepresent specification;

FIG. 17H is a flow diagram representing the steps of using the verticalWSS detector and a ground-level WSS detector shown in FIG. 17G in atransmission mode with a flying spot X-ray imager for scanning a cargo,in accordance with an embodiment of the present specification;

FIG. 17I illustrates a WSS detector panel placed underneath a car tocollect transmission radiation emitted by a small portable scanner beingused to scan a boot of the car, in accordance with an embodiment of thepresent specification;

FIG. 17J illustrates explosives hidden approximately two feet under theboot of the car shown in FIG. 171 ;

FIG. 17K illustrates a transmission image obtained by the WSS detectorpanel placed under the car as shown in FIG. 17I, in accordance with anembodiment of the present specification;

FIG. 17L illustrates a perspective view of the transmission detectorpanel shown in FIG. 17I, in accordance with an embodiment of the presentspecification;

FIG. 17M illustrates a top view of the transmission detector panel shownin FIG. 17I, in accordance with an embodiment of the presentspecification;

FIG. 17N illustrates an exploded view of a plurality of components ofthe transmission detector panel shown in FIG. 17L, in accordance with anembodiment of the present specification;

FIG. 17O illustrates a WSS detector panel placed to collect backscatterradiations emitted by a small portable scanner being used to scan a car,in accordance with an embodiment of the present specification;

FIG. 17P illustrates explosives hidden approximately two feet under theboot of the car shown in FIG. 17O;

FIG. 17Q illustrates a backscatter image obtained by a built-in detectorof the scanner of FIG. 17O, in accordance with an embodiment of thepresent specification;

FIG. 17R illustrates a backscatter image obtained by the scanner and thedetector panel of FIG. 17O, in accordance with an embodiment of thepresent specification;

FIG. 18A illustrates a portable detector for handheld imaging, as usedin a transmission mode, in accordance with an embodiment of the presentspecification;

FIG. 18B illustrates a portable detector for handheld imaging, as usedin a forward scatter mode, in accordance with an embodiment of thepresent specification;

FIG. 18C illustrates a portable detector for handheld imaging, as usedin a back scatter mode, in accordance with an embodiment of the presentspecification;

FIG. 18D is a flowchart illustrating the steps of imaging a target in aforward scatter mode by using a WSS detector panel and a handheldimaging system, in accordance with an embodiment of the presentspecification;

FIG. 19 is a table conveying exemplary dimensions and perimeterfractions of a plurality of WSS detectors in various modes of operation,in embodiments of the present specification.

FIG. 20 illustrates a diagrammatical representation of an object beingscanned by a portable scanner and a WSS detector panel, in accordancewith an embodiment of the present specification;

FIG. 21A illustrates a standard shipping box comprising an explosiveobject not visible with the naked eye;

FIG. 21B illustrates an image of the shipping box obtained by using aportable scanning system and a WSS detector panel of the presentspecification operating in a backscatter mode, in accordance with anembodiment of the present specification;

FIG. 21C illustrates an image of the shipping box obtained by using aportable scanning system and a WSS detector panel of the presentspecification operating in a transmission mode, in accordance with anembodiment of the present specification;

FIG. 22A illustrates explosives hidden in a concrete block;

FIG. 22B illustrates an image of the concrete block shown in FIG. 22A,obtained by using a portable scanning system and a WSS detector panel ofthe present specification operating in a backscatter mode, in accordancewith an embodiment of the present specification; and

FIG. 22C illustrates an image of the concrete block shown in FIG. 22A,obtained by using a portable scanning system and a WSS detector panel ofthe present specification operating in a transmission mode, inaccordance with an embodiment of the present specification.

DETAILED DESCRIPTION

In an embodiment, the present specification discloses an X-ray detectorfor a flying spot transmission imaging system, wherein the detectorenables improved spatial uniformity and reduced cost for materials andmanufacturing.

Definitions

The term “image” shall refer to any unidimensional or multidimensionalrepresentation, whether in tangible or otherwise perceptible form, orotherwise, whereby a value of some characteristic (such as fractionaltransmitted intensity through a column of an inspected object traversedby an incident beam, in the case of X-ray transmission imaging) isassociated with each of a plurality of locations (or, vectors in aEuclidean space, typically R2) corresponding to dimensional coordinatesof an object in physical space, though not necessarily mapped one-to-onethereonto. An image may comprise an array of numbers in a computermemory or holographic medium. Similarly, “imaging” refers to therendering of a stated physical characteristic in terms of one or moreimages.

Terms of spatial relation, such as “above,” “below,” “upper,” “lower,”and the like, may be used herein for ease of description to describe therelationship of one element to another as shown in the figures. It willbe understood that such terms of spatial relation are intended toencompass different orientations of the apparatus in use or operation inaddition to the orientation described and/or depicted in the figures.

Where an element is described as being “on,” “connected to,” or “coupledto” another element, it may be directly on, connected or coupled to theother element, or, alternatively, one or more intervening elements maybe present, unless otherwise specified.

For purposes of the present description, and in any appended claims, theterm “thickness,” as applied to a scintillation detector, shallrepresent the mean extent of the detector in a dimension along, orparallel to, a centroid of the field of view of the detector. The termarea, as applied to a detector, or, equivalently, the term “active area”shall refer to the size of the detector measured in a plane transverseto centroid of all propagation vectors of radiation within the field ofview of the detector.

As used herein, and in any appended claims, the term “large-areadetector” shall refer to any single detector, or to any detector module,subtending an opening angle of at least 30° in each of two orthogonaltransverse directions as viewed from a point on an object undergoinginspection, equivalently, characterized by a spatial angle of at least πsteradians.

A “conveyance” shall be any device characterized by a platform borne onground-contacting members such as wheels, tracks, treads, skids, etc.,used for transporting equipment from one location to another.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting. In the description andclaims of the application, each of the words “comprise” “include” and“have”, and forms thereof, are not necessarily limited to members in alist with which the words may be associated. It should be noted hereinthat any feature or component described in association with a specificembodiment may be used and implemented with any other embodiment unlessclearly indicated otherwise. The singular forms “a,” “an,” and “the,”are intended to include the plural forms as well.

The present specification is directed towards multiple embodiments. Thefollowing disclosure is provided in order to enable a person havingordinary skill in the art to practice the specification. Language usedin this specification should not be interpreted as a general disavowalof any one specific embodiment or used to limit the claims beyond themeaning of the terms used therein. The general principles defined hereinmay be applied to other embodiments and applications without departingfrom the spirit and scope of the specification. Also, the terminologyand phraseology used is for the purpose of describing exemplaryembodiments and should not be considered limiting. Thus, the presentspecification is to be accorded the widest scope encompassing numerousalternatives, modifications and equivalents consistent with theprinciples and features disclosed. For purpose of clarity, detailsrelating to technical material that is known in the technical fieldsrelated to the specification have not been described in detail so as notto unnecessarily obscure the present specification.

The X-ray detector disclosed in the present specification comprises ascreen fabricated from a scintillator material, such as phosphor, thatis optically coupled, in optical contact or in physical communicationwith a wavelength-shifting sheet (WSS), which shifts light absorbed fromthe scintillator screen. The wavelength shifting sheet is coupled to awavelength shifting fiber or sheet at the edge of the wavelengthshifting sheet that is configured to collect a plurality of firstshifted rays. The rays collected from the edge are transmitted throughto a photodetector, such as a photo multiplier tube (PMT).

In various embodiments, the use of a Wave Shifting Sheet (WSS) as theprimary material for flying spot transmission X-ray detection reduces oreliminates objectionable, highly visible defects which are inherent inX-ray detectors implemented with solely Wave Shifting Fibers (WSF). Inaddition, the WSS detector can be fabricated using low cost plasticmaterials assembled in a simple manufacturing process. The WSS detectorof the present specification can be used in conjunction with any flyingspot x-ray system as a transmission, backscatter and forward scatterdetector, and may also include multiple layers for materialsdiscrimination.

FIG. 4A illustrates a diagrammatical representation of a WSF detectorpanel. FIG. 4B illustrates a diagrammatical front and side view of theWSF fibers coupled in a 4x4 array and being held by a mechanicalfixture. As shown in FIGS. 4A and 4B, a plurality of WSF fibers 402 arecoupled with a scintillator screen 408 such that the fibers 402 arepositioned between the scintillator screen 408 and a photomultipliertube (PMT) 406. The fibers 402 held by a mechanical support 404 at theedges and fibers 402 are in contact with each other without having anyspace therebetween. In order to reduce the number of fibers required inthe detector by half, a single fiber loops back from the mechanicalsupport 404 to the PMT 406 causing said fibers to bend as shown in FIG.4A. In various embodiments, fiber ends may be trimmed and polished toimprove optical transmission and response uniformity. Optical grease maybe used to improve the optical transmission from the fiber to the PMT.FIG. 4C illustrates the WSF detector panel of FIG. 4A, in accordancewith an embodiment of the present specification. The detector panel ispreferably made of off-the-shelf (OTS) materials, and the constituentWSF fibers are held together without any spacing between them.

FIG. 4D illustrates a graph 410 illustrating the fiber responseuniformity of the detector panel shown in FIGS. 4A and 4B. An example ofthe uniformity across the detector (from fiber to fiber) is plotted inFIG. 4D. The greyscale plot 410 depicts the signal across the detector.In this case, variability is created due to the groups of bends in thefiber at the ends of the detector as shown in FIG. 4A. The variabilityis enough to create artifacts which are visible during imaging, cannotbe corrected or calibrated, and could be objectionable to the userduring typical imaging. Plot 410 also depicts the output light intensityas a function of input X-ray beam position as measured across thedetector. The light response output has 80% variability as calculated bythe difference between the maximum and minimum intensity divided by themean.

Conventionally, wave-shifting fibers are cut and their ends are polishedfor coupling to a PMT surface. FIG. 5 shows the bundled end of 1 mmwave-shifting fibers 502 which have been cut and polished for couplingto a PMT surface. Any non-uniformity in the polishing of the bundle fromfiber to fiber will lead to light loss in an individual fiber. The lightloss will create a line defect across the entire image detected by thePMT.

Wavelength Shifting Sheet (WSS) Detector Structure

FIG. 6A illustrates a Wave Shifting Sheet (WSS) X-ray detector, inaccordance with an embodiment of the present specification. FIG. 6B is aside view of the WSS detector shown in FIG. 6A. Referring to both FIGS.6A and 6B, WSS detector 600 comprises a sheet 602 fabricated from awavelength shifting material and at least partially covered by ascintillator screen 604 on at least a portion of, or the entirety of, afirst surface 602 a of a first side or at least a portion of, or theentirety of, a second surface 602 b of a second side, or preferably boththe first surface 602 a of the first side and the second surface 602 bof the second side, of sheet 602, as shown in FIGS. 6A and 6B. Invarious embodiments, the sheet 602 is a contiguous WSS sheet, and ischaracterized by a single solid media with single thickness throughoutits surface. In an embodiment, sheet 602 is not comprised of a solidribbon or a ribboned bundle of wavelength shifting fibers. In anembodiment, sheet 602 has a smaller aspect ratio compared to aconventional WS ribbon. In an embodiment, the first surface 602 a of thefirst side or the second surface 602 b of the second side, or preferablyboth the first surface 602 a of the first side and the second surface602 b of the second side of sheet 602, is smooth and the surfaces arecoplanar with minimal defects in order to maintain total internalreflection and minimize the loss of light. In various embodiments,defects may include any surface imperfections such as scratches, pits,surface particles, fibers or surface bumps/burrs; as well as defectsinterior to the sheet including bubbles, captured particles or fibers.In embodiments, the surfaces of the sheet 602 are polished and free ofscratches, pits, or imperfections. In an embodiment, the surfaces arepolished to less than 0.5 µm RMS roughness with less than 10 isolateddefects of size up to 100 µm in size in a total area of 1 cm², less than3 isolated defects of size ranging from 100 µm to 500 µm in size in atotal area of 1 cm², and no defects are larger than 500 µm.

Typically, in a ribboned bundle of WS fibers, the individual fibers havean aspect ratio of 1 mm x 2 mm which is constrained to fit the area of atypical PMT. In an embodiment, for a horizontal detector sheet, atypical circular fiber diameter is 1 mm.

In the WS sheet of the present specification, the aspect ratio may beconstrained to a thickness ranging from 1 mm to 10 mm and a widthranging from 2 mm to 3,000 mm. In an embodiment, the thickness of the WSsheet 602 ranges from 2 mm to 10 mm and the width ranges from 2 mm to 5mm in order to capture a majority of the light exiting the sheet.

In an embodiment, the scintillator screen 604 is a phosphor screen. Inan embodiment, the border edge 606 of the wavelength shifting materialsheet 602 comprises a wavelength shifting fiber(s) 607 which, in turn iscoupled to a photodetector, such as a photomultiplier tube (PMT) 608. Inan embodiment, a second WS material sheet is used in place of the WSfiber(s) 607. Thus, in an embodiment, the edge 606 of the wavelengthshifting material sheet 602 may comprise a second wavelength shiftingmaterial or a wavelength shifting fiber(s), which may be different fromthe material of WS sheet 602. During operation of the X-ray detector600, the scintillator screen 604 absorbs any incident X-rays and emitscorresponding light rays which are then shifted by the sheet 602. Theshifted light is collected by the wavelength shifting (WS) fiber 607provided at the edge 606. The shifted light is then shifted again by theWS fiber 607 and transmitted through to the PMT 608 for subsequentdetection. Since, in this embodiment, only two fibers (one from a firstside and one from a second side) enter the PMT 608, a required PMT areais less than that required with prior art detectors.

In an embodiment, the second WS material used in edge 606 of the WSS 602has a refractive index of 1.5 or greater to improve a capture efficiencyof total internal reflection of light which is generated by fluorescenceinside the material. In various embodiments, the WS material used inedge 606 absorbs light from the WS sheet 602 and fluoresces at a longerwavelength, and efficiently trapping fluorescent light. In embodiments,the WSS detectors of the present specification are manufacturing usingpolyvinyl butyral for sheet 602 and polystyrene (n=1.6) with PMMAcladding for the wavelength shifting fiber 607.

FIG. 6C is a diagrammatical representation of a WS detector, showinglight shifted by a second WS material in a direction perpendicular to anedge of a WS detector, in accordance with an embodiment of the presentspecification. In various embodiments, a light ray 650 which is shiftedby the sheet 602 is absorbed by the second WS material of edge 606 at alocation 606 a, and re-directed in a direction 651 perpendicular to aline 652 that is normal to location 606 a as shown in FIG. 6C. The WSsheet 602 may have an irregular shaped edge, however the light rayredirection is always normal to the edge location at which the light rayis received after it is redirected by sheet 602.

In various embodiments, the highest capture efficiency for the WS sheet602 occurs when the media surrounding the sheet 602 has an index of 1(air), and the index of the sheet 602 is as high as possible. Hence, inembodiments, the WSS detector 600 is obtained by placing a scintillatorscreen 604 over a wavelength shifting sheet 602 to maintain an air gapbetween the scintillator 604 and the WSS 602, and as such, does notrequire an embedding or molding process, thereby decreasing the cost ofmanufacture. If an adhesive is used to couple the sheet scintillator 604and the WS sheet 602, a low-index adhesive is beneficial, because, as isknown, the total fraction of fluorescent light collected by an infiniteWS sheet without scattering corresponds to the following formula:

$\text{F=}\sqrt{\left( {1 - \frac{\text{n}_{1}{}^{2}}{\text{n}_{2}{}^{2}}} \right)}$

where n₁ is the index of refraction of the surrounding media and n₂ isthe index of refraction of the WS sheet medium. Hence, in someembodiments, small regions of adhesive material may be used to attachthe scintillator screen, with the contact area remaining as small aspossible.

In various embodiments, the scintillator screen 604 does not cover theentire surface of the WS sheet 602. In embodiments, a scintillatorscreen 604 smaller than the WS sheet 602 may be coupled with the WSsheet 602 in such a manner that the scintillator screen 604 is not incontact with the WS sheet near the edges 606, in order to improveuniformity of the image obtained by using the detector 600. If thescintillator screen 604 is placed close to the edge of the WSS 602,there is a greater amount of direct exposure to the edge fiber(s). Thescintillator screen 604 absorbs any incident X-rays and emitscorresponding light rays which are then shifted by the sheet 602 andthen absorbed by the WS fiber 607 at the edge 606 of the sheet 602 for asecond shift. If scintillation screen 604 is placed close to the edge ofthe WSS 602, the photons from light emitted due to scintillation are soclose to the fiber(s), that they are directly absorbed by the WS fiber607, without being shifted by the WS sheet 602.

FIG. 6D is a pictorial representation of an image obtained by placing ascintillator screen close to the edge of the WSS. Bright regions 630 atthe edge region are caused by direct illumination (not WSS shifted)scintillator light striking the WSF. Hence, in various embodiments, thescintillator screen 604 is of a different size than WSS 602. In anembodiment, the scintillator screen 604 is either cut smaller than theWSS, or an opaque material is applied along the edges of thescintillator screen to block out a portion of the scintillator screenedge.

Since the light transmitted to the PMT 608 exits from a narrow region610 along the edge of the WSS detector 600, in an embodiment, the PMT608 may be included in the same enclosure as the WSS detector 600, ormay be coupled through a clear fiber optic cable to an external PMT (notshown in the FIG. 6A) having an area equal to or larger than the exitarea of the cable. In an embodiment, having a coupling with a cable fromboth ends of the detector to minimize cable absorption losses, anexternal PMT having an area equal to approximately 20 mm² is used for atotal WS sheet thickness of 3 mm to 5 mm and a width of an edge borderWS fiber at approximately 2 mm, for absorption of 90% of the incidentlight.

In various embodiments, the areal density of the top and bottomscintillating screen 604 can be optimized for maximum x-ray absorption.

FIG. 7A illustrates a diagrammatical side view of a WSS X-ray detector,in accordance with an embodiment of the present specification. FIG. 7Billustrates a diagrammatical front view of the WSS X-ray detector shownin FIG. 7A. As shown a first set of incident X rays 702 are absorbed bya top scintillator layer 704 and the corresponding emitted light 703enters the WSS layer 706, which in turn shifts the light 703 a firsttime and transmits the light outward, to its edges, in all directionssuch that the light 703 is received, absorbed, and then shifted a secondtime by WS fibers 708. A second set of incident X rays 710 which passthrough the top scintillator layer 704 and the WSS layer 706, areabsorbed by a bottom scintillator layer 712 and the corresponding lightre-enters the WSS layer 706, which in turn shifts the light andtransmits the light outward, to its edges, in all directions such thatthe light is also absorbed by WS sheet or fibers 708. The shifted lightabsorbed by the WS sheet or fibers 708 and shifted again are transmittedalong the length of the WS sheet or fibers and subsequently transmittedinto a photo multiplier tube (PMT) 714 for detection. In an embodiment,a second WS material sheet is preferably used in place of the WS fibers708.

An advantage of the second shift of light by the WS sheet or fibers 708is that the photons are concentrated into a smaller exit face area. Witha single shift as used in most currently available detectors, the PMTarea is proportional to the width of the detector multiplied by itsthickness. A typical detector width is 400 mm and a typical thickness ofa WS fiber is 1 mm, leading to a required PMT area of 400 mm². By usinga WS sheet instead of fiber and thus incorporating a second shift, thePMT area needed reduces to the thickness of the WS sheet multiplied bythe width of the second shift region or edge region. A typical sheetthickness is 2 mm and a typical width of the second WS material 708 is 2mm, which equals a required PMT area of 4 mm², with a significantreduction by a factor of 100X. The area reduction factor is limited byboth a light trapping efficiency and a fundamental physical limit whichprevents light concentration where the total entropy is reduced.

In an embodiment, the second WS material 708 covers the entire face ofthe first WS sheet 706. Further in an embodiment, the efficiency of thelight collection in the WS sheet 708 is improved by increasing thethickness of the second WS material 708. FIG. 7C illustrates adiagrammatical side view of a WSS X-ray detector, wherein the thicknessof the second WS material 708 is increased, in accordance with anembodiment of the present specification. As shown in FIG. 7C, theadditional thickness captures light 705 which exits the second WSmaterial 708 at an angle equal to the critical angle of the material. Inan embodiment where the width and thickness of the second WS material708 is ‘W’ and ‘T’, respectively, the optimized value of ‘T’ forcapturing of 90% of the light emitted by WS sheet 706 is:

T2 = T1 + 2 ⋅ cos(α_(c))

where ‘αc’ is the critical angle for first WS sheet 706, and sin(αc)=1/n; where ‘n’ is the refractive index for second WS material 708,assuming air is surrounding second WS material 708. In embodiments wherethe second WS material 708 is fiber, the light capture efficiency isdependent on the shape of the fiber, and a greater capture efficiencymay be obtained by using a square cross section of WS fiber 708.

In an embodiment, a reflector material 716 is provided around allexposed edges of WS fiber or sheet to improve X ray absorption andtransmission to the PMT 714. The reflector material 716 may be composedof a diffuse reflector (paint or tape) or a specular reflector material(metallic).

In various embodiments, the thickness of the top scintillator layer 704does not exceed the optical absorption length of the scintillatormaterial, as that may lead to absorption of X-rays which do notcontribute to the signal detected by PMT 714. As is known, the opticalpath length in a scintillator screen 704 is limited by absorption oflight scattered in the phosphor. Hence, X-rays which are absorbed nearthe top of the scintillator screen 704, emit light which is absorbedbefore exiting the bottom of the scintillator screen 704 and enteringthe WSS 706.

The scintillator material emits visible light, preferably in the UVportion of the spectrum in order to maintain efficient energy transfer.In an embodiment, Europium-doped barium fluorochloride (BaFCI:Eu) isused as the scintillator material. In other embodiments various othersuitable scintillator materials, such as, but not limited to GadoliniumOxysulfide, and Cesium Iodide may be used.

There are advantages to collecting light shifted by the WS sheets with awavelength-shifting fiber 607 used at the edges of the sheet 602, asopposed to using a second wavelength shifting sheet, including thefollowing:

-   WS fibers are not directly illuminated in the active area of the    image, and thus variations in individual fibers response (output    coupling, bend loss, fiber defects) affect only large regions of the    image and not lines across the image;-   WS fibers enable more efficient collection around the corners of the    detector; and,-   WS fibers are flexible and the coupling cable can be extended past    the edge of the active area of the detector to enable remote    coupling to the PMT at a large distance from the detector (up to 1    meter).

In an embodiment, the WS sheet of the detector as described withreference to FIGS. 7A, 7B, and 7C is optically segmented into pixelsusing a mechanical milling process. FIG. 7D is a block diagram of asegmented WSS of a WSS X-ray detector array, in accordance with anembodiment of the present specification. WSS 720 comprises a pluralityof segments 722 formed by milling grooves 732 into WSS 720, bound by WSfibers 724 at the edges. In various embodiments, the grooves have athickness ranging from 0.25 mm to 1 mm and a depth ranging from 50% to90% of the total sheet thickness are cut into WS sheet having a minimumthickness of 0.5 mm. In various embodiments, the milled regions 732(dead region) are made as thin as possible to maintain detectorefficiency. FIG. 7E illustrates grooves 732 milled into the surface of aWS sheet 720, in accordance with an embodiment of the presentspecification. FIG. 7F is a block diagram illustrating a front view ofgrooves milled into the surface of a WS sheet, in accordance with anembodiment of the present specification. As shown in FIGS. 7D, 7E and7F, WS sheet 720 is divided into a plurality of segments 722 by grooves732. In embodiments, the grooves 732 may be filled with reflectormaterial such as, but not limited to, 3M ESR tape, Teflon tape, or whiteTiO₂ paint to prevent cross talk of optical photons between pixels whilealso improving detector efficiency. To maintain the mechanical structureof the WSS 720 when the grooves are placed into the WS material, thesheet may be adhesively bonded to a light-weight and thin mechanicalsupport 734. In embodiments, the top of the mechanical support 734 iscoated with a reflector material 736 to reflect scintillator light intothe sheet. The minimum pixel size, which in an embodiment is 0.5 mm islimited by the machining of the grooves. If the pixel size isapproximately the same as the groove width, the dead region in the pixelwill be large and the detector will not function. The minimum pixel sizemay be approximately 2 mm, for a 0.5 mm groove width. As described withreference to FIGS. 7A and 7D, the segmented pixelated WS sheet iscovered with a scintillator layer for converting incident X rays intolight rays which are collected by a WS fiber provided at the edge of theWSS detector and fed to a PMT. FIG. 7G illustrates a two-step spectraloverlap in a WSS detector having a segmented WSS, in accordance with anembodiment of the present specification. Plot 750 illustrates absorptionand emission plots of both a scintillator and WSS while plot 752illustrates absorption and emission plots of both the WSS and WSF. Plot750 demonstrates the light spectrum overlap between the emission spectraof the scintillator and the absorption of the WSS. Plot 752 demonstratesthe spectral overlap between the WSS and the WS fiber. Regions where thetwo curves do not overlap represent inefficiency in the light couplingbetween the detector layers.

FIG. 8A is a graph depicting an absorption and emission spectra ofBaFCI:Eu in the WSS layer of the detector shown in FIG. 7A. FIG. 8B is agraph depicting an absorption and emission spectra of BaFCI:Eu in the WSfiber layer of the detector shown in FIG. 7A. As show in FIG. 8A,scintillator material BaFCI:Eu has an emission spectrum peak near 390nm. Light is absorbed in the WSS layer and emitted at the shiftedwavelength of 400 nm.

With reference to FIGS. 7A and 7B light 703 travels in the WSS detectorthrough either diffuse reflection from the surface of the WS sheet 706or the scintillator layer 704, or through total internal reflection(TIR) from the WSS surface. All light rays which are emitted in the WSsheet 706 at an angle which is larger than the critical angle of the WSmaterial are captured by TIR and propagated to the edge of the sheet706. Hence the light collection efficiency of the WSS detector isdependent on the index of refraction of the WS sheet 706.

In an embodiment, in order to improve the TIR angle, the index ofrefraction of the scintillator material 704 maybe decreased while theindex of the WSS layer 706 may be increased, as the critical angle

$\theta_{c} = \arcsin\left( \frac{n_{2}}{n_{1}} \right)\mspace{6mu};$

where n₁ denotes the refractive index of the WSS material 706 and n₂denotes the refractive index of air if no adhesive is used to attach thescintillator 704 to the WSS. For example, if typical plastics such asacrylic/PMMA are used in the detector, having a refractive index n1=1.4and wherein the refractive index of air is equal to 1, the criticalangle θ_(c),can be calculated as being equal to 40°.

It is advantageous to use WS sheets for obtaining X ray detectors asprovided by the present specification instead of using WS fibers, as thelight collection in WS sheets is more efficient than that in WS fibers.FIG. 9A illustrates light collection cones in a WS fiber. In a WS fiber902, light rays which fall within two light collecting cones 904 aretransmitted via TIR along the length of the fiber 902. FIG. 9Billustrates light loss cones in a WS sheet. In sheet 906, light iscollected for all rays except for light lost due to rays which fallwithin two light loss cones 908, leading to a much wider volume ofacceptance angles.

FIG. 9C is a diagrammatical representation of light collection cones ina rectangular WS fiber. As shown in FIG. 9C rectangular fiber 910comprises four light collecting cones 912, one cone corresponding toeach of the four surface of fiber 910.

The solid angle for a single light loss cone is

$\text{Ω}_{\text{cone}} = 2\pi\left( {1 - \sqrt{1 - \frac{1}{\text{n}^{2}}}} \right)$

Thus the capture efficiency for the rectangular WS fiber 910 is:

$\text{F}_{\text{rect}} = \frac{\left( {4\pi - 4 \cdot \text{Ω}_{\text{cone}}} \right)}{4\pi} = \left( {2 \cdot \sqrt{1 - \frac{1}{\text{n}^{2}}}} \right) - 1$

Assuming that an infinite WS sheet has two faces, and thus two lightloss cones have a capture efficiency of:

$\text{F}_{\text{sheet}} = \sqrt{\left( {1 - \frac{1}{n^{2}}} \right)}$

Hence, it can be seen that the light capture efficiency in WS sheets isgreater than that in WS fibers. FIG. 9D is a graphical representation ofthe light capture efficiencies of a WS Sheet 920 and a WS fiber 930 madeof materials having the same refractive index. As, demonstrated by theplots 920 and 930, WS sheets provide better light capturingefficiencies.

Referring to FIGS. 7A and 7B again, light will be attenuated in the WSSmaterial 706 which contains a dye material, leading to a uniformdecrease in the signal intensity away from the X-ray source absorptionpoint. The dye material is used to absorb light emitted from thescintillator and emit light that transmits to the edge of the WS sheet706. In various embodiments, any commercially available wavelengthshifting paint may be used as the dye material. In order to minimizeabsorption, the WSS 706 material is kept as thin as possible forcreating a shortest length path for the light 703 to reach to the edgeof the sheet. Conversely, the WSS 706 material thickness is increased tominimize the number of reflections from the surface of the sheet.Depending on the attenuation length of light in WSS 706 material and theabsorption loss in the scintillator layer 704, the optimal thickness ofthe WSS 706 changes. For example, the thickness of a WSS 706 materialmay range from 2 mm to 10 mm, with an attenuation length ofapproximately 3.5 cm.

In edge region, the light 703 is absorbed and re-emitted for collectionby the PMT 714. In an embodiment, in order to improve couplingefficiency and uniformity, the edge WSS or WSF 708 material is incontact with the WSS 706 material around four sides of the detector; andthe width of the edge WSS or WSF 708 material is increased to an optimallevel for better absorption of light from the WSS 706. In embodiments,the width of the edge WSS or WSF 708 material is determined by measuringthe optical attenuation length at the wavelength of emission light fromthe WSS 706, and selecting a length with an attenuation greater than90%. In various embodiment, the width of the edge WSS or WSF 708material ranges from 2 mm to 5 mm. In an embodiment, the width of theedge WSS or WSF 708 material is 4 mm.

FIG. 10A illustrates a WSS detector comprising a wavelength shiftingsheet 1002 coupled with a wavelength shifting fiber (WSF) wrappedaround/couple to/in direct communication with four edges of the sheet,in accordance with an embodiment of the present specification. In otherembodiments, the WSF may be wrapped around/coupled to/in directcommunication with one, two, or three edges/sides of the WS sheet 1002.In the embodiment shown in FIG. 10A, the WS sheet has a dimension of 12″x 12″ and the aggregate WSF fiber at the edges comprises a bundle of sixWS fibers. The corners of the of the WS sheet 1002 have been rounded foruniform bending of the WSF fiber with a fillet having an approximatediameter of 0.75”.

In an embodiment, the detector 1000 is constructed using a plastic (PVB)WSS material coated with a dye for absorbing light in the UV spectrumand emitting light in the blue spectrum, and having a width ranging from50 mm to 800 mm, height ranging from 50 mm to 2,500 mm and thicknessranging from 2 mm to 10 mm; wherein all edges of the sheet are diamondmilled.

In an embodiment, a scintillator screen of BaFCI:Eu having a densityranging from 40 mg/cm2 to 250 mg/cm2 is coupled with the front and backfaces of WSS detector 1000. In an embodiment a reflector tape 1004 isused to attach the WSS 1002 with the WSF, as well as to attach thescintillator screen to the WSS. A PMT 1006 is coupled with the detector1000 for signal detection. In other embodiments, the WS sheet may becircular, oval, or have an irregular shape with at least one edge;wherein, the WSF is wrapped around at least a portion of the edge.

FIG. 10B illustrates image data obtained from the detector shown in FIG.10A. The image is a transmission image of a HDPE phantom with resolutionblocks demonstrating steel penetration. Penetration thru ⅜″ thick steelis demonstrated in the outset portion 1030 of the image 1040. The WSSdetector 1000 as described in the present specification has apredictable and repeatable non-uniform response. FIG. 10C illustratesthe gain uniformity of the image response of the WSS detector shown inFIG. 10A. As shown, excluding the edge region, the detector has a smoothvarying gain with a variability being greater than 10%. FIG. 10Dillustrates a log-linear cross-cut of the signal response shown in FIG.10C.

When coupled with a second WS material on all four sides, thenon-uniformity of the detected image obtained by using the WSS detectoras described above, is symmetric and lowest in the center of thedetector panel with a weak change due to attenuation in the second WSmaterial sheet. In an embodiment, the non-uniformity is corrected byinserting a spatially varying attenuating material between thescintillator screen and the WSS. FIG. 11A is a block diagram depictingan attenuating material inserted between WSS and a scintillator screen,in accordance with an embodiment of the present specification. Referringto FIG. 11A attenuating material 1102 is inserted between WSS 1104 and ascintillator screen 1106 in the form of a compensating sheet. Theattenuating material 1102 may be applied in the form of a printed sheetwith absorbing ink on the surface. For a given set of materials andsizes of detector, the non-uniformity and compensation pattern are fixedand repeatable from detector to detector. The compensating sheet made ofthe attenuating material 1102 is printed with a pattern that attenuateshigh response regions of the detector panel. In various embodiments, theattenuating material 1102 is a spatially varying attenuating material,used to smooth out the repeatable gain variations in the WS sheet.Repeatable variations may be caused by the geometry of the WSS, by thelight cross-shading in the detector gain. In embodiments, by insertingthe compensating sheet with attenuation of light emitted from thescintillator, the gain variation can be corrected.

FIG. 11B illustrates an image showing the response uniformity of the WSSdetector, in accordance with an embodiment of the present specification.FIG. 11C illustrates a compensating pattern of the attenuating material1102 used in the WSS detector shown in FIG. 11A. In various embodiments,the spatially attenuating material 1102 has the followingcharacteristics:

-   Dynamic range & Extinction coefficient: For maximum absorption, the    attenuating material has a large extinction coefficient with the    ability to absorb 100 x the total light output from the scintillator    in 1-2 mm thickness (µ=5 cm-1). The spectral absorption is neutral    (flat) across the range from 350 nm to 600 nm in order to cover the    full range of scintillators commonly used. The minimum absorption    cuts the output of the scintillator by <6-7% of the total output.-   Low cost: <100 \$ for 16″ x 16″ part, substrate plastic or other    readily available material.-   Printed/Patterned: ability to change absorption with printing    resolution of at least 0.5 mm, using dithered printing or greyscale.-   Thin: total attenuator material is less than 1-2 mm thick in order    to add a minimum of thickness to the WSS detector.

In various embodiments, techniques such as but not limited to laserprinted pattern on acetate film by using either greyscale pattern ordithered printing; printed absorber such as: screen printing, inkjetprinting, transfer print, print inks on acetate or other clearsubstrate, or directly print absorber on the WSS; absorbing sheet withvarying thickness; textured sheet or textured surface of WSS such asbead blast, sanded, chemically roughened, or direct transfer thru mold,or plastic film or directly textured WSS surface; printed scintillatormaterial; and variable dye concentration WSS such as spatially patternedsurface printed dye and resin on the WS sheet may be used.

As described above with reference to FIG. 7A, in various embodiments, itis advantageous collect of X rays with WS fibers as opposed to a secondWS material sheet. FIG. 12A illustrates an image showing signal responsefrom a WS fiber edge coupled WSS detector. FIG. 12B illustrates agraphical depiction of a cross cut of the signal response shown in FIG.12A, horizontally across the center region of the WSS detector. Invarious embodiments, the response of a WSS detector coupled with foursided WSF collection shows an improvement in uniformity. The uniformityimproves from a 100X variation in a single sided WSS to 2.6X variationfrom edge to center of the detector in a WSS detector coupled with foursided WSF collection. In addition, variations are smooth and correctablewith an absorbing filter. In embodiments, the number of fibers coupledwith the edges of a WSS detector may be increased in order to improvethe total amount of light absorbed. FIG. 12C illustrates additional WSFibers 1202 arranged at the edge of WSS 1204 and a scintillator screen1206, in accordance with an embodiment of the present specification. Invarious embodiments, inserting additional fibers increases the totalamount of light collected at the edge of the sheet 1204.

FIG. 12D shows a plot depicting a WSS detector response with variablenumber of fibers used for light collection at the edge of the WS sheet,in accordance with an embodiment of the present specification. Plot 1222depicts response from a WSS detector having 2, 4 and 6 WS Fibers coupledwith the edges. Plots 1224, 1226 and 1228 depict response from WSSdetectors coupled along a single edge with 2, 4 and 6 WS fibersrespectively for feeding collected X-rays to a PMT. As shown by theplots, additional fibers add to the amount of material which can absorblight at the edge of the sheet, and in embodiments, each additional twofibers adds 1 mm of thickness to the absorbing region thickness.

FIG. 13A is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to foursides of a wavelength shifting sheet enabling a four side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification. WSS detector 1340 comprises a scintillatorscreen 1342 partially covering a WS sheet 1344, which is in turn coupledwith WS Fibers 1346 on all four sides for transmitting shifted lightrays into a PMT 1348. In an embodiment, the scintillator screen 1342 isan 80 mg/cm2 sheet and is applied on both sides (top and bottom) of theWS sheet 1344 which has a thickness of 4 mm with corners rounded at0.75” ROC. In an embodiment, the WS fibers 1346 are provided in the forma cable having a diameter of 1 mm, and the PMT 1348 has a diameter of 8mm.

FIG. 13B is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/couple to twosides of a wavelength shifting sheet, enabling a two side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification. WSS detector 1350 comprises a scintillatorscreen 1352 partially covering a WS sheet 1354, which is in turn coupledwith two WS fiber cables 1356 on two sides (as shown in FIG. 13B) fortransmitting shifted light rays into a PMT 1358. In an embodiment, theremaining two sides of the WS sheet 1354 are covered with a reflectormaterial 1359 to improve X-ray absorption.

FIG. 13C is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to oneside of a wavelength shifting sheet, enabling a single side readout asused in a transmission detection mode, in accordance with an embodimentof the present specification. WSS detector 1370 comprises a scintillatorscreen 1372 partially covering a WS sheet 1374, which is in turn coupledwith one WS fiber cable 1376 on one sides (as shown in FIG. 13C) fortransmitting shifted light rays into a PMT 1378. In an embodiment, theremaining three sides of the WS sheet 1374 are covered with a reflectormaterial 1379 to improve X-ray absorption.

FIG. 14A is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to foursides of a wavelength shifting sheet, enabling a four side as used in abackscatter detection mode, in accordance with an embodiment of thepresent specification. WSS detector 1440 comprises a scintillator screen1442 partially covering a WS sheet 1444, which is in turn coupled withWS fibers 1446 on all four sides (as shown in FIG. 14A) for transmittingshifted light rays into a PMT 1448. In embodiments, the WS fibers 1446may be provided in the form of a single cable.

FIG. 14B is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to twosides of a wavelength shifting sheet, enabling a two side readout asused in a backscatter detection mode, in accordance with an embodimentof the present specification. WSS detector 1450 comprises a scintillatorscreen 1452 partially covering a WS sheet 1454, which is in turn coupledwith two WS fiber cables 1456 on two sides (as shown in FIG. 14B) fortransmitting shifted light rays into a PMT 1458. In an embodiment, theremaining two sides of the WS sheet 1454 are covered with a reflectormaterial 1459 to improve X-ray absorption. In an embodiment, the WSfibers 1446 may be provided as two cables.

FIG. 14C is a diagrammatical representation of a rectangular WSSdetector having WS fibers in direct communication with/coupled to oneside of a wavelength shifting sheet, enabling a single side readout asused in a backscatter detection mode, in accordance with an embodimentof the present specification. WSS detector 1470 comprises a scintillatorscreen 1472 partially covering a WS sheet 1474, which is in turn coupledwith one WS fiber cable 1476 on one sides (as shown in FIG. 14C) fortransmitting shifted light rays into a PMT 1478. In an embodiment, theremaining three sides of the WS sheet 1474 are covered with a reflectormaterial 1479 to improve X-ray absorption.

In an embodiment, a WSS detector operating in backscatter mode comprisesa plurality of channels cut into a WS sheet wherein WS fibers arepositioned within said channels instead of along one or more sides ofthe WS sheet. FIG. 14D is a diagrammatical representation of arectangular WSS detector having discrete WS fibers placed within aplurality of corresponding channels cut into the WS sheet, as used in abackscatter detection mode, in accordance with an embodiment of thepresent specification. WSS detector 1480 comprises a scintillator screen1482 partially covering a WS sheet 1484, comprising channels, eachchannel housing a WS fiber 1486 (as shown in FIG. 14D) for transmittingshifted light rays into a PMT 1488. In embodiments, eight channels andthus, eight WS fibers 1486 are employed. In an embodiment, a reflectormaterial 1490 is provided around all exposed edges of WS sheet 1484 toimprove X ray absorption and transmission to the PMT 1488.

FIG. 15A is a side view of a backscatter WSS detector comprising aplurality of channels for housing WS fibers, in accordance with anembodiment of the present specification. FIG. 15B is a top view of thebackscatter WSS detector comprising a plurality of channels for holdingWS fibers, in accordance with an embodiment of the presentspecification. The placement of WS fibers in channels cut within thesurface of the WS sheet is advantageous as a smaller length of WS fibermay be required, compared to embodiments where the WS fiber is placedalong the sides of the WS sheet. In an embodiment, the spacing betweenthe channels ranges between 5 mm to 20 mm; and for a 20 cm x 20 cm x 0.5cm sheet with channels at 1 cm spacing, approximately (20 cm * 20channels) + 20 cm = 420 cm of WS fiber is required; in comparison,(2*(20+20))*(8 cables) = 640 cm of WS fiber being required for anembodiment where the WS fiber is placed along the sides of a WS sheet ofthe same dimensions.

Co-pending U.S. Pat. Application No. 16/242,163, of the same Applicantof the present specification, entitled “Spectral Discrimination usingWavelength-Shifting Fiber-Coupled Scintillation Detectors” is hereinincorporated by reference in its entirety. In addition, U.S. Pat.Application No. 15/490,787, entitled “Spectral Discrimination usingWavelength-Shifting Fiber-Coupled Scintillation Detectors”, filed onApr. 18, 2017, which, in turn, is a divisional application of U.S. Pat.No. 9,658,343 (the “‘343 patent”), of the same title filed on Feb. 23,2016 and issued on May 23, 2017 are also incorporated by referenceherein in their entirety. Also, U.S. Pat. No. 9,285,488 (the ‘488patent), of the same title, filed on Feb. 4, 2013, and issued on Mar.15, 2016 and any priority applications thereof are herein incorporatedby reference in their entirety. The embodiments described in the presentspecification are more cost-effective as they do not require thatindividual fibers are separated into low and high resolution, with eachneeding separate PMTs, which would increase the overall length of thefiber. Thus, the wavelength-shifting sheet scintillation detectors ofthe present specification are lower cost owing to both the eliminationof a fiber bundle and manufacturing complexity due to the handling,cutting and polishing of the fiber bundles included therein.

As shown in FIGS. 15A and 15B, a first set of incident X rays 1502 areabsorbed by a top scintillator layer 1504. The corresponding emittedlight 1503 enters a WS sheet 1506, which in turn shifts the light afirst time, such that the shifted light 1507 is received, absorbed, andshifted a second time by WS fibers 1508 placed within channels 1509formed within the WS sheet 1506. A second set of incident X rays 1510which pass through the top scintillator layer 1504 and the WSS 1506, areabsorbed by a bottom scintillator layer 1512 whereby the correspondingemitted light re-enters the WSS 1506, which in turn shifts the light1515 a first time, such that the shifted light is received, absorbed,and shifted a second time by WS fibers 1511 placed within channels 1513formed within the WS sheet 1506. The light shifted by the WS fibers1508, 1511 is transmitted along the length of the WS fibers 1508, 1511and subsequently transmitted to a photo multiplier tube (PMT) 1514 fordetection. In embodiments, a reflector material 1516 is provided aroundall exposed edges of WS sheet 1506 to improve X ray absorption andtransmission to the PMT 1514. The reflector material 1516 may becomposed of a diffuse reflector (paint or tape) or a specular reflectormaterial (metallic) such as, but not limited, to Teflon tape, 3Menhanced specular reflector (ESR) Tape and Silver Metalized Polyester.

In an embodiment, along with having channels housing wavelength shiftingfibers as described with reference to FIGS. 15A and 15B, the sheet of aWSS detector in accordance with an embodiment of the presentspecification, may be optically segmented into pixels using a mechanicalmilling process. FIG. 15C is a diagrammatical representation of across-section of a WSS detector array with a segmented pixelated WSsheet comprising a plurality of channels for holding WS fibers, inaccordance with an embodiment of the present specification. FIG. 15D isa top view of the WSS detector having two WS fibers placed withinchannels cut into the segmented WS sheet, in accordance with anembodiment of the present specification. As shown in FIG. 15C, ascintillator layer 1580 converts incident X rays 1582 from a flying spotX ray beam 1581 to scintillation light 1584 which is typically in theultraviolet (UV) wavelength range of approximately 400 nm. The light1584 then falls upon a segmented WS sheet 1586 comprising opticallyisolated pixels ranging in size from approximately 2 mm to 5 mm. Asshown in FIG. 15C the WSS 1586 comprises a plurality of channels 1590containing WSF 1592. In an embodiment, the segments 1588 and the WSF1592 are made perpendicular to each other. The WSS 1586 converts the UVlight 1584 to blue light having a wavelength of approximately 425 nmwhich then strikes WSF 1592, which in turn absorbs the blue light andconverts it to green light having a wavelength of approximately 475 nm.The green light is detected by a PMT connected to the WSF 1592.

Light from the pixelated WSS 1586 is multiplexed at the transition fromWSS 1586 to WSF 1592. In an embodiment, a reflective material coating isinserted between the WSS 1586 and the WSF 1592 for preventing light fromexiting the WSS 1586. In an embodiment, the channels 1590 containing theWSF 1592 are not covered by the reflective coating. In an embodiment,the reflective coating may be patterned so that only light from theselected channel may exit the WSS. In this manner, pixels may be groupedto share a signal on a common fiber, if the X-ray beam spot 1581 doesnot simultaneously illuminate two pixels from the same segment.

In various embodiments, a reflective coating may be patterned onto theedge of the WSS 1586 or directly onto the WS fiber 1592, so that onlylight from specific channels may enter the fiber. FIG. 15C illustrates areflector deposited on the edge of the WSS 1586 everywhere except at theopenings for the channels. Light from the fibers exits at the PMT.Hence, in various embodiments, light is either reflected or transmittedto the WSF 1592 using the optical reflector coating. Thus, pixels of thesame multiplexed group can transmit light from the WSS 1586 to the WSF1592.

In some imaging applications, for example non-destructive pipelineinspection, it is important to maintain proximity from the object beingimaged to the detector in order to prevent a degradation of the spatialresolution of the recorded image. Hence, in an embodiment, the WSSdetector is made of a curved WS sheet. FIG. 16 illustrates a curved WSSdetector, in accordance with an embodiment of the present specification.As shown, a curved scintillator screen 1602 at least partially covers acurved WS sheet 1604 having a WSF cable coupled along the edges forenhancing the uniformity of the detector.

In an embodiment, a typical thickness of a WSS detector is 6 mm, whereintwo scintillator screens having a thickness of 1 mm each, a WSS having athickness of 2 mm and a cover having a thickness of 3 mm is employed. Inan embodiment, the WSS detector has an area greater than 432 mm*432 mm;weight less than 2.5 kg; and a bezel thickness less than 4 mm. In anembodiment, the WSS detector operates in a temperature range of -20C to50C, and a humidity range of 20% to 80%.

In an embodiment, the WSS detector of the present specification may beconverted into a multi-energy detector. The multi-energy WSS detectormay be obtained by employing a layered structure that includes a highenergy and a low energy WSS detector. A filter such as, but not limitedto, a sheet of copper may be inserted between two stacked WSS (high andlow energy) detectors to obtain a multi-energy WSS detector.

In another embodiment, the WSS detector described in the presentspecification may be converted to a flexible X-ray detector by using WSSand/or edge collection WSF/WSS made of flexible materials such as, butnot limited to silicone based materials. In embodiments, where thephotodetector available for coupling with the WSS detector is largerthan the thickness of the active area of the WSS detector, the PMT maybe removed from the WSS detector package and coupled with the detectorvia a clear fiber optic cable.

Wavelength Shifting Sheet (WSS) Detector Implementation

In various embodiments, the WSS detector described above may beimplemented as a transmission detector, a forward scatter detector, anda backscatter detector, depending on the placement of the detector withrespect to an imager being used in conjunction with the detector. Inembodiments, the detector of the present specification is implemented inan imaging system having an enclosure and housing that is built aroundthe imaging system, and that is separate from the imaging system.

FIG. 17A is a diagrammatic representation of a WSS detector as used in atransmission mode with a flying spot X-ray imager, in an embodiment ofthe present specification. A flying spot X-ray imager 1702 irradiates anobject 1704 being inspected with a flying spot beam 1706, as shown inthe FIG. A WSS detector 1708 placed behind the object 1704 (such thatthe object 1704 is between the imager 1702 and the detector 1708),receives a transmission beam 1710 comprising X rays that are transmittedthrough the object 1704. The beam 1710 is absorbed by a scintillatorlayer (not shown in the FIG.) and shifted by one or more WS materials ofthe WSS detector 1708 as explained in the preceding sections. Theshifted beam is transmitted to a photomultiplier tube 1712 for detectionwhich in turn transmits the detected data to a data acquisition system(not shown in the FIG.) of the imager 1702 for processing.

FIG. 17B is a flow diagram representing the steps of using an WSSdetector in a transmission mode with a flying spot X-ray imager, inaccordance with an embodiment of the present specification. At step 1701an object under inspection is irradiated with a flying spot X-rays,emanating from an X-ray imager. At step 1703 a transmission beamcomprising X rays that are transmitted through the object are receivedby a WSS detector placed behind the object (such that the object ispositioned between the imager and the detector). At step 1705, thetransmission beam is absorbed by a scintillator layer of the detectorand emitted as corresponding light rays. At step 1707, the emitted lightrays are shifted by one or more WS materials of the WSS detector. Atstep 1709, the shifted light rays are transmitted to a PMT fordetection. At step 1711, the shifted light rays are converted toelectrical signals by the photomultiplier tube. At 1713, the electricalsignals are transmitted to a data acquisition system for processing.

FIG. 17C is a diagrammatic representation of a WSS detector as used in abackscatter mode with a flying spot X-ray imager, in an embodiment ofthe present specification. A flying spot X-ray imager 1702 irradiates anobject 1704 being inspected with a flying spot beam 1706, as shown inthe FIG. A WSS detector 1708 placed before the object 1704 (such thatthe detector 1708 is between the imager 1702 and the object 1704),receives a backscatter beam 1720 comprising X rays that arebackscattered by the object 1704. The beam 1720 is absorbed by ascintillator layer (not shown in the FIG.) and shifted by one or more WSmaterials of the WSS detector 1708 as explained in the precedingsections. The shifted beam is transmitted to a photomultiplier tube 1712for detection which in turn transmits the detected data to a dataacquisition system (not shown in the FIG.) of the imager 1702 forprocessing. In the embodiment shown in FIG. 17B, the WSS detector 1708is implemented as an auxiliary backscatter detector while the imager1702 comprises a primary backscatter detector 1722 integrated with theimager 1702. In embodiments, the auxiliary detector provides additionaldetection areas, capturing more of the scattered X-ray photons and thusboosting the contrast to noise ratio (CNR) of the imaging system.

FIG. 17D is a flow diagram representing the steps of using a WSSdetector in a backscatter mode with a flying spot X-ray imager, inaccordance with an embodiment of the present specification. At step 1721an object being inspected is irradiated with a flying spot X-rays,emanating from an X-ray imager. At step 1723 a backscatter beamcomprising X rays that are backscattered by the object are received by aWSS detector placed before the object (such that the detector ispositioned between the imager and the object). At step 1725, thebackscatter beam is absorbed by a scintillator layer of the detector andemitted as corresponding light rays. At step 1727, the emitted lightrays are shifted by one or more WS materials of the WSS detector. Atstep 1729, the shifted light rays are transmitted to a PMT fordetection. At step 1731, the shifted light rays are converted toelectrical signals by the photomultiplier tube. At 1733, the electricalsignals are transmitted to a data acquisition system for processing.

FIG. 17E is a diagrammatic representation of a multi-energy WSS detectoras used in a transmission mode with a flying spot X-ray imager, in anembodiment of the present specification. A flying spot X-ray imager 1702irradiates an object 1704 being inspected with a flying spot beam 1706,as shown in the FIG. A multi-energy WSS detector 1730 comprising alayered structure that includes a high energy 1732 and a low energy 1734WSS detector, with a filter 1736 such as, but not limited to, a sheet ofcopper inserted between the two stacked WSS (high and low energy)detectors is placed behind the object 1704 (such that the object 1704 isbetween the imager 1702 and the detector 1730), receives a transmissionbeam 1710 comprising X rays that are transmitted through the object1704. The beam 1710 is shifted by each of the high energy 1732 and thelow energy 1734 WSS detectors and the shifted beams are transmitted tophotomultiplier tubes 1738 and 1740 respectively for detection, which inturn transmit the detected data to a data acquisition system (not shownin the FIG.) of the imager 1702 for processing. The filter 1736 providesa shift in the energy of the transmission beam 1710, preferentiallyremoving more low energy X-rays and shifting the beam spectrum to ahigher energy level. As a result, in various embodiments the firstdetector layer is a Low Energy (LE) detector 1734 and the second postfilter 1736 layer is a High Energy (HE) detector 1732.

Multi-energy WSS detectors are described in U.S. Pat. No. 9,285,488entitled “X-ray inspection using wavelength-shifting fiber-coupledscintillation detectors”, assigned to the Applicant of the presentspecification, and is herein incorporated by reference in its entirety.Multi-energy detectors work by separating the signals from both thefront and back layers of scintillators, which allows the front layer togive a measure of the low-energy component of each pixel while the backlayer gives a measure of the high-energy components. Putting a layer ofabsorbing material between the front and back scintillators is astandard methodology to enhance the difference between low and highenergy components, which is typically done with a Sc-WSF detector. TheSc-WSF detector makes practical a dual-energy detector consisting of alayer of Sc-WSF, such as BaFCl-WSF, on top of a plastic scintillatordetector; the BaFCl is sensitive to the low-energy x-rays and not thehigh-energy x-rays, while the plastic detector is sensitive to thehigh-energy x-rays and very insensitive to low energy x-rays. Analternative and potentially more effective material discriminator can bemade by using more than two independent layers of Sc-WSF, with separatereadouts for each layer. A passive absorber, such as an appropriatethickness of copper, can be inserted after the top Sc-WSF to enhancedual energy application, as is practiced with segmented detectors.Alternatively, the middle scintillator can be used as an activeabsorbing layer. The measurement of three independent parameters allowsone to get a measure of both the average atomic number of the traversedmaterials and the extent of beam hardening as well.

FIG. 17F is a flow diagram representing the steps of using themulti-energy WSS detector shown in FIG. 17E, in a transmission mode witha flying spot X-ray imager, in accordance with an embodiment of thepresent specification. At step 1741 an object being inspected isirradiated with a flying spot X-rays, emanating from an X-ray imager. Atstep 1743 a transmission beam comprising X rays that are transmittedthrough the object are received by the multi-energy WSS detectorcomprising a high energy (HE) and a low energy (LE) WSS material layer,with a filter inserted between the two stacked WSS layers, placed behindthe object (such that the object is positioned between the imager andthe detector). At step 1745, the transmission beam is absorbed by ascintillator layer of the detector and emitted as corresponding lightrays. At step 1747, the emitted HE light rays are shifted by the HE WSSmaterial layer of the WSS detector and are transmitted to a first PMTcoupled with the HE WSS material layer. At step 1749, the emitted LElight rays are shifted by the LE WSS material layer of the WSS detectorand are transmitted to a second PMT coupled with the LE WSS materiallayer. At step 1751, the shifted light rays are converted to electricalsignals by the first and the second PMTs. At 1753, the electricalsignals are transmitted to a data acquisition system for processing.

FIG. 17G is a diagrammatic representation of a vertical WSS detector anda ground-level WSS detector as used in a transmission mode with a flyingspot X-ray imager for scanning a cargo object, in an embodiment of thepresent specification. A flying spot X-ray imager 1752 irradiates acargo object 1754 being inspected with a flying spot beam 1756, as shownin the FIG. A WSS vertical detector 1758 and a WSS ground detector 1760are arranged perpendicular to each other, as shown in FIG. 17G forscanning the cargo object 1754. A WSS vertical detector 1758 and a WSSground detector 1760 are placed behind the object 1754 (such that theobject 1754 is between the imager 1752 and the detectors 1758, 1760),receive a transmission beam 1770 comprising X-rays that are transmittedthrough the object 1754. The beam 1770 is absorbed by a scintillatorlayer (not shown in the FIG.) and shifted by one or more WS materials ofthe WSS detectors as explained in the preceding sections. The shiftedbeam is transmitted to a photomultiplier tube 1772 for detection whichin turn transmits the detected data to a data acquisition system (notshown in the FIG.) of the imager 1752 for processing. WSS verticaldetector 1758 may have multiple detector segments in a directiontransverse to the direction of transmission beam 1770 and substantiallyalong the direction of relative motion between inspected cargo object1754 and transmission beam 1770 so as to provide an indication ofskewness or lateral shift of the detectors with respect to the beam.

FIG. 17H is a flow diagram representing the steps of using the verticalWSS detector and a ground-level WSS detector shown in FIG. 17G in atransmission mode with a flying spot X-ray imager for scanning a cargo,in accordance with an embodiment of the present specification. At step1761 the cargo being inspected is irradiated with a flying spot X-rays,emanating from an X-ray imager. At step 1763 a transmission beamcomprising X rays that are transmitted through the cargo are received bythe WSS vertical detector and the WSS ground detector arrangedperpendicular to each, and placed behind the cargo (such that the cargois positioned between the imager and the detector). At step 1765, thetransmission beam is absorbed by scintillator layers of the WSS verticaldetector and the WSS ground detector and are emitted as correspondinglight rays. At step 1767, the emitted light rays are shifted by one ormore WS materials of the WSS vertical detector and the WSS grounddetector. At step 1769, the shifted light rays are transmitted to one ormore PMTs coupled with the WSS vertical detector and the WSS grounddetector for detection. At step 1771, the shifted light rays areconverted to electrical signals by the one or more PMTs. At 1773, theelectrical signals are transmitted to a data acquisition system forprocessing.

In embodiments, the detector of the present specification may be used inan integrated mode or an accessory mode with respect to an imagingsystem. The embodiments described above are representative ofimplementation in an integrated mode. In embodiments, in accessory mode,the WSS detector of the present specification further includes anenclosure and housing is built around the detection system that isseparate from an imaging system where the imaging system includes aradiation source. The detection system enclosure comprises at least ahandle and is powered by a power source built within the enclosure or ispowered by an external power source. In an embodiment, the WSS detectionsystem being used in an accessory mode is self-powered and wireless. Inan embodiment, the detected radiation converted to electrical signals bya PMT of the WSS detection system is conveyed to the imaging system forprocessing via a shielded cable in an analog form. In anotherembodiment, the analog electrical signals are converted to digitalsignals and conveyed to the imaging system wirelessly. In an embodiment,the detected radiation may also be conveyed to the imaging system aslight signals, wherein the PMT electronics for processing the lightsignals is provided in the imaging system, thereby reducing the size ofthe WSS detector system significantly by removing all PMT electroniccomponents from the detection system. An advantage of using the WSSdetector as a small portable accessory to the imaging system is that theposition of the detector with respect to an object being scanned can bevaried easily, thereby optimizing scan coverage of the object.

In some embodiments, the scanning system being used in conjunction withthe WSS detector of the present specification comprises a plurality ofchannels for coupling with one or more detectors. The channels may beconfigured via a user interface to enable simultaneous coupling withmore than one WSS detectors. In embodiments, the detector-type is userconfigurable. For example, a system may have a built-in backscatterdetector configuration and two additional detector channels that may beuser configured. The images captured by each of said detectors mayeither be processed separately or may be summed.

During the use of the WSS detector in a transmission mode, there is awide range around the detector’s WSS panel, where the imaging isapparent through forward scatter interaction of X-rays irradiating theobject being scanned. Hence, by using the WSS detector in an accessorymode, users may position the detector in desirable orientations withrespect to the object.

FIG. 17I illustrates a detector panel placed under a car to collecttransmission radiations emitted by a small portable scanner being usedto scan a boot of the car, in accordance with an embodiment of thepresent specification. As shown, detector panel 1782 is placed under aboot 1784 of the car to collect the transmission radiation emitted bythe portable scanner 1786 which is being used to scan the boot 1784.FIG. 17J illustrates explosives hidden approximately two feet under theboot 1784 of the car shown in FIG. 17I. The explosives include explosivestimulants i.e. 5 lbs of sugar 1788 and 500 ml liquid 1790 and a gun1792. FIG. 28C illustrates a transmission image 1794 obtained by thedetector panel placed under the car as shown in FIG. 17I, in accordancewith an embodiment of the present specification. As can be seen in FIG.17K, the transmission image 1794 clearly shows the metal gun 1792 whilethe organic explosives 1788, 1790 become nearly transparent in theimage. Hence even though, when used as a transmission detector, theactive size of the detector panel 1782 may become limiting, the forwardscatter generated by the scanner 1786 in the vicinity of the detector1782 is also detected, thereby making the effective field of view largerthan the detector area.

FIG. 17L illustrates a top perspective view of a prototype of thetransmission detector panel shown in FIG. 17I, in accordance with anembodiment of the present specification. FIG. 17M illustrates a top viewof a prototype of the transmission detector panel shown in FIG. 17I, inaccordance with an embodiment of the present specification. FIG. 17Nillustrates a close up view of the components of the transmissiondetector panel shown in FIG. 17L, in accordance with an embodiment ofthe present specification. Referring to FIGS. 17L, 17M and 17Ntransmission detector panel 1795 comprises WS sheet 1796 coated with atop layer of a scintillator material 1797, and is bounded by WS fibercables 1798. During a detection operation, the scintillator material1797 absorbs the transmission X rays falling on the detector panel 1795and converts said rays to light rays, which in turn are shifted by theWSS 1796 and WS fiber 1798 and fed to a PMT 1799 powered by a powersupply 1791. The WS fiber cables 1798 are placed in cable retainers 1787and are coupled with the PMT 1799 via fiber cable guide 1789 and fibercollet 1793.

FIG. 17O illustrates a WSS detector panel placed to collect backscatterradiation emitted by a small portable scanner being used to scan a car,in accordance with an embodiment of the present specification. As shown,detector panel 17102 is placed to collect the backscatter radiationemitted by the portable scanner 17104 which is being used to scan theboot 17106 of a car. FIG. 17P illustrates explosives 17108 hiddenapproximately two feet under the boot 17106. FIG. 17Q illustrates abackscatter image 17110 obtained by a built-in detector of the scannerof FIG. 17O, in accordance with an embodiment of the presentspecification. The backscatter image 17110 obtained by the scanner17104, which is obtained using the in-built detectors (not shown in theFIG.) of said scanner 17104 does not show the hidden explosive 17108(shown in FIG. 17P). FIG. 17R illustrates a backscatter image 17112obtained by the scanner and the detector panel of FIG. 17O, inaccordance with an embodiment of the present specification. As can beseen in FIG. 17R, the backscatter image 17112 clearly shows theexplosive stimulant 17108 hidden within the boot 17106. The spatialresolution of the backscatter image 17112 is governed by the scanningbeam spot size, however, the beam penetration and SNR is greatlyenhanced as compared to the backscatter image 17110.

FIG. 18A illustrates a portable detector for handheld imaging, as usedin a transmission mode, in accordance with an embodiment of the presentspecification. FIG. 18B illustrates a portable detector for handheldimaging, being used in a forward scatter mode, in accordance with anembodiment of the present specification. FIG. 18C illustrates a portabledetector for handheld imaging, as used in backscatter mode, inaccordance with an embodiment of the present specification. Referring toFIGS. 18A, 18B and 18C, in various embodiments, in order to operateportable WSS detector 1806 for handheld imaging in transmission, forwardscatter or backscatter mode, an operator 1802 is required to positionthe detector 1806 with respect to the target being scanned 1808,energize a hand-held imaging system 1804, translate the imaging system1804 across the region of the target 1802 and collect the scattered andtransmission data for review. Referring to FIG. 18A, transmitted photonsare detected by the detector 1806. Referring to FIG. 18B, the detectorpanel 1806 is placed on a side opposite a side of the hand held imagingsystem 1804 with respect to the target 1808, or, in some embodiments, atright angles to the target 1808, such that when the target 1802 isirradiated with X rays from the imaging system 1804 the forward scatterfrom the target 1802 is detected by the WSS detector panel 1806.Referring to FIG. 18C backscattered photons are detected by thehand-held imaging system 1804.

FIG. 18D is a flowchart illustrating the steps of imaging a target in aforward scatter mode by using a WSS detector panel and a handheldimaging system, in accordance with an embodiment of the presentspecification. At step 1810 a WSS detector panel is placed on a sideopposite a side of a hand held imaging system with respect to the targetobject being scanned. In various embodiments, the detector panel isplaced in close proximity to the target as shown in FIG. 18B. At step1812, the target is irradiated with X rays from the hand held imagingsystem. At step 1814, forward scatter from the target is captured anddetected by the detector panel. In various embodiments, the detectedforward scatter is processed by an image processing system coupled withthe detector panel to produce a scanned image of the target object.

FIG. 19 is a table conveying the dimensions and perimeter fractions of aplurality of WSS detectors, in an embodiment of the resentspecification. The perimeter fraction is the amount of the edge wherethe WSF cable is in contact with the WSS. Row 1902 illustrates thedimension and perimeter fraction of a handheld transmission WSSdetector, wherein the length ranges from 8 to 16 inches, width rangesfrom 8 to 24 inches, and perimeter fraction ranges from 50% to 100%. Row1904 illustrates the dimension and perimeter fraction of a handheldbackscatter WSS detector, wherein the length and width ranges from 6 to12 inches, and perimeter fraction ranges from 25% to 50%. Row 1906illustrates the dimension and perimeter fraction of a cargo backscatterWSS detector, wherein the length ranges from 40 to 80 inches, widthranges from 15 to 40 inches, and perimeter fraction ranges from 50% to100%. Row 1908 illustrates the dimension and perimeter fraction of acargo transmission WSS detector, wherein the length ranges from 80 to150 inches, width ranges from 6 to 12 inches, and perimeter fractionranges from 50% to 100%.

Hence, the present specification provides a WSS detector for use in aflying spot transmission imaging system with improved spatialuniformity. The cost for materials and manufacturing the WSS is less ascompared to prior art detectors as: off-the-shelf scintillating screensmay be used in place of specialized molded parts; there is norequirement for fiber handling, bundling or polishing; and adhesives arenot required in any of the active optical paths, greatly reducing thechances for yield failure due to bubbles or voids.

Since, backscatter systems have imaging limitations in certainapplications where Transmission imaging is advantageous, the WSSdetector of the present specification is implemented as a TransmissionScan Panel that can be used with any commercially available handheldscanning system such as, but not limited to, the MINI Z scanning system,to provide a simultaneous secondary image. As described above, thedetector of the present specification provides a transmission image whenplaced directly behind an item being scanned, or an additionalbackscatter image when placed on the near side of the object beingscanned. The secondary image is displayed next to the standardBackscatter image on a display screen coupled with the MINI Z scanningsystem. In various embodiments, the Transmission Scan Panel is portable,light-weight, and connects to handheld scanning system with a simple,single cable connection.

In an embodiment, some exemplary physical attributes of the WSS detectorpanel of the present specification are:

-   Active Imaging Area: 16 in x 22 in (41 cm x 56 cm)-   Weight: 10 pounds (4.5 kg)-   Cable length: Up to 30 feet (9 m) total, using 3 individual 10 foot    (3 m) cables connected in series.

In an embodiment, some exemplary performance specification of the WSSdetector panel of the present specification are:

-   Nominal Penetration: up to 20 mm steel-   Nominal Resolution: 0.5 mm at 0 cm standoff; 3 mm at 10 cm standoff;    and 4 mm at 25 cm standoff

In an embodiment, the WSS detector panel of the present specificationincludes a manual gain adjust reduce/eliminate image noise.

FIG. 20 illustrates a diagrammatical representation of an object 2002being scanned by a portable scanner 2004 and a WSS detector panel 2006,in accordance with an embodiment of the present specification. In anembodiment, where the object 2002 is a wire-resolution phantom object, astandoff distance 2008 between the object 2002 and the imaging system2004 is 0 cm to obtain a 1.5 mm wire resolution; 3 mm to obtain a 10 cmwire resolution; and 5 mm to obtain a 25 cm wire resolution.

FIG. 21A illustrates a standard shipping box 2102 comprising anexplosive object not visible to naked eye. FIG. 21B illustrates an image2104 of the shipping box obtained by using a portable scanning systemand a WSS detector panel of the present specification operating in abackscatter mode, in accordance with an embodiment of the presentspecification. As shown in FIG. 21B the explosive object which is a pipebomb is not visible in the scanning image 2104 obtained by operating theWSS detector panel in a backscatter mode. FIG. 21C illustrates an image2106 of the shipping box obtained by using a portable scanning systemand a WSS detector panel of the present specification operating in atransmission mode, in accordance with an embodiment of the presentspecification. As shown in FIG. 21C the pipe bomb 2108 is clearlyvisible in the scanning image 2106 obtained by operating the WSSdetector panel in a transmission mode.

FIG. 22A illustrates explosives hidden in a concrete block. As can beseen in the figure, a soda can IED 2202 and a steel pipe bomb 2204 arehidden in a concrete block 2206, with approximately 1.5 inches thickwalls. FIG. 22B illustrates an image 2210 of the concrete block shown inFIG. 22A, obtained by using a portable scanning system and a WSSdetector panel of the present specification operating in a backscattermode, in accordance with an embodiment of the present specification. Asshown in FIG. 22B the explosive objects are not visible in the scanningimage 2210 obtained by operating the WSS detector panel in a backscattermode. FIG. 22C illustrates an image 2212 of the concrete block shown inFIG. 22A, obtained by using a portable scanning system and a WSSdetector panel of the present specification operating in a transmissionmode, in accordance with an embodiment of the present specification. Asshown in FIG. 22C the soda can IED 2202 and a steel pipe bomb 2204 areclearly visible in the scanning image 2212 obtained by operating the WSSdetector panel in a transmission mode.

The above examples are merely illustrative of the many applications ofthe system and method of present specification. Although only a fewembodiments of the present specification have been described herein, itshould be understood that the present specification might be embodied inmany other specific forms without departing from the spirit or scope ofthe specification. Therefore, the present examples and embodiments areto be considered as illustrative and not restrictive, and thespecification may be modified within the scope of the appended claims.

We claim:
 1. An X-ray detector comprising: at least one scintillatorscreen configured to absorb incident X rays and emit corresponding lightrays; a wavelength shifting sheet (WSS) optically coupled with the atleast one scintillator screen and configured to collect and spectrumshift the light rays, wherein the WSS has at least one edge; at leastone wavelength shifting fiber (WSF) optically coupled with the at leastone edge of the WSS and configured to collect the spectrum shifted lightrays and spectrum shift the collected spectrum shifted light rays togenerate twice-spectrum-shifted light rays; and a photodetectoroptically coupled to the WSF and configured to receive and detect thetwice-spectrum-shifted light rays.
 2. The X-ray detector of claim 1wherein the WSS comprises a first and a second surface and wherein theat least one scintillator screen at least partially covers the firstsurface and a second scintillator screen at least partially covers thesecond surface.
 3. The X-ray detector of claim 2 wherein the firstsurface is coplanar to the second surface.
 4. The X-ray detector ofclaim 1 wherein the at least one WSF is physically coupled with at leasta portion of the edge of the WSS.
 5. The X-ray detector of claim 1wherein the photodetector is a photomultiplier tube (PMT).
 6. The X-raydetector of claim 1 further comprising a reflector material covering theWSF to improve the collection of the spectrum shifted light rays.
 7. TheX-ray detector of claim 6 wherein the reflector material comprises atleast one of a diffuse reflector or a specular reflector material. 8.The X-ray detector of claim 1 wherein the at least one scintillatorscreen comprises a material having an optical absorption length andwherein a thickness of the at least one scintillator screen is less thanthe optical absorption length.
 9. The X-ray detector of claim 1 whereinthe at least one scintillator screen comprises BaFC1:Eu.
 10. The X-raydetector of claim 1 further comprising a spatially varying attenuatingmaterial inserted between the at least one scintillator screen and theWSS, wherein the spatially varying attenuating material is configured tocorrect a non-uniformity in detection by the photodetector.
 11. TheX-ray detector of claim 10 wherein the spatially varying attenuatingmaterial comprises a plastic substrate printed sheet with absorbing inkon a surface of the plastic substrate printed sheet.
 12. The X-raydetector of claim 1 wherein the at least one scintillator screen iscoupled with the WSS by placing the at least one scintillator screenover a surface of the WSS and wherein the at least one scintillatorscreen at least partially covers the surface.
 13. An X-ray detectorconfigured to detect X-rays, the detector comprising: at least onescintillator screen configured to absorb incident X rays and emit lightrays based on the absorbed incident X-rays; a first wavelength shiftingsheet (WSS1) coupled with the at least one scintillator screenconfigured to shift spectra of the light rays, wherein the WSS1comprises at least one edge; a second wavelength shifting sheet (WSS2)coupled with at the least one edge of said WSS1 and configured tocollect the spectra shifted light rays; and a photodetector configuredto detect the collected spectra shifted light rays, wherein thephotodetector is in optical communication with the WSS2.
 14. The X-raydetector of claim 13 wherein the WSS1 comprises a first and a secondsurface; and wherein the at least one scintillator screen partiallycovers the first surface and a second scintillator screen partiallycovers the second surface.
 15. The X-ray detector of claim 14 whereinthe first surface is coplanar to the second surface.
 16. The X-raydetector of claim 13 wherein the photodetector is a photomultiplier tube(PMT).
 17. The X-ray detector of claim 13 further comprising a reflectormaterial covering the WSS2 to improve the collection of the spectrashifted light rays.
 18. The X-ray detector of claim 17 wherein thereflector material comprises at least one of a diffuse reflector or aspecular reflector material.
 19. The X-ray detector of claim 13 whereinthe at least one scintillator screen comprises a material having anoptical absorption length and wherein a thickness of the scintillatorscreen is less than the optical absorption length.
 20. The X-raydetector of claim 13 wherein the at least one scintillator screen ismade of BaFCI:Eu.